The card states they suffer wounds, is there no defense roll?

No. Defence rolls are only involved in response to attacks.

daa

A bit different question about it. Can it be used at the end of the round in which the droid has been deployed?

A while ago my rebel players were in a situation where the last unwounded hero [in Imperial Hospitality] was standing next to the green spawn with 5 HP left. I had two probe droid deployment cards in my hand and enough threat to play for them both, so I figured I will play them and detonate them in the same round [as the deployment and reinforcement step is before the end of round effects and 5 dmg on two red dice is almost a sure thing after all], but my players started arguing if it is a legal play [since they have no option to react to it]. After going through the rules I found nothing saying it is allowed or it is not allowed, so I decided that in the spirit of good fun I won't do it this time [after all we are still learning to play]. Unfortunatelly I lost the mission in the process. So I would like to hear your opinions on that matter. I think it is a legal play, but I would like to know for sure.

as the deployment and reinforcement step is before the end of round effects

Fair play!

You were fortunate enough to have probe droids in your hand and the rebel players were unfortunately next to a spawn point.

A very rare situation, but legal!

It's completely legal... unfortunately. You deploy your reinforcements and then you go through end of round effects. This order contains a lot of things like this, any time there are mission specific end of round effects you can get the surprise on the Rebels by deploying the thing that receives the effect, Royal Guards do this in a couple of missions I think. But Probe Droid deploy/self-destruct is why you wanna keep tabs on where the Imperial deploys his troops. And if he uses a deployment he's never used before, you're just SOL.

Also importtant to remember the order of things when missions cause the IP to gain additional threat at the end of a round

Also importtant to remember the order of things when missions cause the IP to gain additional threat at the end of a round

Indeed, this is a very abused error, meaning that a lot of Imperials make it. A lot of newer Imperial Players will see "Gain 4 threat." under an End of Round condition, see that everyone is finished moving, will advance the threat dial by the additional amount and then deploy with it.

1. Advance Threat Dial.

2. Deploy.

3. End of Round Effects <--- is where the additional threat would be added.

So they have to wait another turn to use it, giving the Rebels time enough to plan against it.